1. Field of the Invention
The present invention relates to a pattern inspection technology, that is, a technology that can.be effective when applied to fault inspection of a pattern on a semiconductor wafer.
2. Description of the Related Art
According to a typical conventional pattern inspection method, multi-value images are acquired while an inspection object is being scanned continuously in an x direction by an image acquiring unit comprising the, combination of an optical microscope with an imaging device such as a TDI (Time Delay Integration) camera, and these images are then stored in an image data storing unit such as a memory. Gray levels of adjacent dies are compared in a pixel unit in parallel with the operation described above, and a pixel having a gray level difference, that exceeds a predetermined reference value, is recognized as a pixel that may indicate faults. The appearance inspection of the entire surface of a semiconductor wafer is conducted by continuously executing these operations.
Acquisition of a high quality image is of utmost importance in order.to achieve high precision inspection in the appearance inspection. The high quality image must have sufficient bright-and-dark contrast, precise focus on the inspection object, and so forth.
An appearance inspection.apparatus xe2x80x9cKLA-2135xe2x80x9d of KLA Co., U.S.A., as a typical example of-the appearance inspection apparatuses that are now commercially available, is equipped with an x-y-z stage capable of moving relatively to a microscope that is kept fixed. This x-y-z stage is controlled in the z direction on the basis of a feedback signal from an auto-focus mechanism so that the gap between the surface of a semiconductor wafer and an objective lens is always constant. Therefore, the altitude in the z direction, that is judged as being in xe2x80x9cjust-in-focusxe2x80x9d by the auto-focus mechanism, is traced during scanning in the x direction.
In the chip structure of a semiconductor device, however, structures having different surface altitudes such as a cell portion, a peripheral circuit portion, etc, are fabricated. For this reason, the rays of light are not always in just focus on the chip surface even when the auto-focus mechanism judges the focusing state as xe2x80x9cjust-in-focusxe2x80x9d. When the semiconductor devices are inspected with a high level of accuracy through chip comparison, it has therefore become difficult in recent years to acquire a high quality image using the appearance inspection apparatus having the construction described above.
In the case of a DRAM as a typical memory device, the chip can be divided broadly into a cell portion and a peripheral circuit portion. In order to secure the capacitance at the cell portion, the cell portion tends to extend upward with miniaturization of the device size in comparison with the peripheral circuit portion.
FIG. 4A of the accompanying drawings is a top plan view showing an example of a known semiconductor device, and FIG. 4B is a sectional view taken along a line IVxe2x80x94IV of FIG. 4A. As shown in these drawings, an altitude difference exists between the cell portion and the peripheral circuit portion, and this difference reaches nearly 1 micron in certain devices.
The appearance inspection apparatus desirably has the capability of detecting faults with a high level of accuracy in both of the cell portion and the peripheral circuit portion.
FIG. 5 is an explanatory view useful for explaining a scanning operation in the transverse direction on the upper surface of the semiconductor device shown in FIG. 4A. In FIG. 5, a reference symbol W represents the width that is covered by the one-time scanning operation by the TDI camera. When scanning is conducted in such a fashion that the two different regions, i.e., the cell portion and the peripheral circuit portion, are contained at the upper and lower portions in the figure of the same scanning range as represented by the region 1 the auto-focus mechanism can place the focus on only one of them. In other words, the other region is subjected to a comparative inspection using an image that is acquired under an unfocused state and has low image quality.
Another problem of the focusing operation is that the image is acquired by the TDI camera. The output of the TDI camera is analogous to that of a one-dimensional line sensor, and it outputs an apparently one-dimensional image information. However, the TDI camera has practically a two-dimensional light reception surface, and makes up for insufficiency of light power resulting from high-speed scanning by serially moving the charge to adjacent pixels in synchronism with the relative movement of the inspection object so as to increase the charge storage quantity. Therefore, while a certain region of the inspection object is being moved in the direction of integration of the TDI camera, a predetermined distance must be kept between the light reception surface of the TDI camera and the image formation surface in order to acquire a high quality image. However, when scanning is made in such a fashion as to bridge over the altitude difference between the cell portion an d the peripheral circuit portion as represented by the region 2 in FIG. 5, the auto-focus mechanism moves the wafer, which is the inspection object, in the z direction so as to place the focus on the boundary. In other words, the overlapping image of the image, which is in focus, and the image, which is out of focus, is outputted in the proximity of the altitude difference portion as represented by the region 2 with the result of the drop of image quality. The region in which deterioration of image quality occurs becomes greater as the number of integration stages by the TDI camera increases. Therefore, the method that makes up for insufficiency of light power by increasing the number of integration stages becomes unfeasible. For these two reasons, the images having the just-in-focus cannot be acquired in all the regions of the entire surface of the chip and eventually, it has been difficult to improve the fault detection sensitivity.
The appearance inspection apparatus with a confocal microscope equipped with an electric scanning table, that is described in European Unexamined Patent Publication No. 0871052 (xe2x80x9cMicroscope with Movable. Scanning Tablexe2x80x9d) laid open on Oct. 14, 1998, has a much smaller depth of focus than ordinary microscopes because the confocal microscope is employed. In inspecting only a region having a specific altitude, this appearance inspection apparatus can obtain excellent effects because the depth of focus is extremely small, while making other regions difficult to observe. In consequence, this appearance inspection apparatus can drastically improve the image contrast of the inspection region and eventually, can inspect the appearance with high sensitivity. However, when it is desired to inspect only the outermost surface of both region of an inspection object having a large altitude difference between, for example, the cell portion and the peripheral circuit portion, this appearance inspection apparatus can naturally place the focus on only either one of these portions and can hardly acquire effective image information of the other portion due to the confocal effect. Therefore, when both of the cell portion and the peripheral circuit portions are to be inspected, it is necessary to conduct the full surface inspection by placing the focus on the cell portion and then to conduct once again the full surface inspection by placing the focus on the peripheral circuit portion. Thus, inspection efficiency drops.
In view of the problems with the prior art technologies described above, it is an object of the present invention to provide an appearance inspection method, and an apparatus therefor, capable of acquiring high quality images having a precise focus over the entire zone of an inspection object.
Typical aspects of the invention described in this application may be briefly summarized as follows.
The appearance inspection apparatus of the present invention combines a light source having two or more wavelength bands with a microscope having chromatic aberration.so that rays of light of one of the wavelength bands can always place the focus on the surface of the inspection object even when the inspection object has the altitude difference in an area between, for example, the cell portion and the peripheral circuit portion. In this way, the appearance inspection apparatus can acquire high quality images having a precise focus throughout all the zones of the inspection object.
According to one aspect of the present invention for accomplishing this object, the present invention provides an appearance inspection method, and an apparatus therefor, that use two TDI cameras each having a sensitivity in only a specific wavelength band in order to simultaneously acquire two images on different focal planes, and slice the images for comparative inspection in accordance with a region division that is defined in advance.
According to another aspect of the present invention, there are provided an appearance inspection method, and an apparatus therefor, that acquire images by using a co-focal microscope having two corresponding pin-hole arrays on the illumination side and on the light reception side, and one TDI camera.
An explanation will be given in further detail. According to one aspect of the present invention, in an appearance inspection method for detecting a fault of an inspection object by comparing an image that is obtained by imaging a pattern arranged on the inspection object in a predetermined direction with an image of a pattern arranged at a corresponding position of other inspection object, the present invention provides an appearance inspection method, and an apparatus therefor, that comprise the steps of radiating rays of light by illumination means capable of radiating rays of light having at least two wavelength bands; changing the direction of the rays of light from the illumination mean by a beam splitter; causing the ray of light, the direction of which is so changed, to focus on the surface of the inspection object through an optical system having such chromatic aberration as to place the focus at a position corresponding to each wavelength band; separating the rays of light of the two wavelength bands reflected from the surface of the inspection object into two directions by a half mirror through an optical system and a beam splitter; causing one of the rays of light so separated to travel towards a first wavelength selection means and the other ray of light so separated to travel towards a second wavelength selection means; allowing one of the rays of light of at least two wavelength bands to pass through the first wavelength selection means and the other of the rays of light of the other wavelength band to pass through the second wavelength selection means; receiving the rays of light passing through the first and second wavelength selection means by at least two imaging means, respectively, so as to thereby image an image of a pattern region for each wavelength band; synthesizing the taken image data so taken; and detecting a fault on the inspection object on the basis of the comparison result obtained by comparing the image of each region of the synthetic image with an image of a pattern arranged at a corresponding position of other inspection object.
Imaging is conducted using a TDI camera.
The optical system having chromatic aberration comprises a plurality of objective lenses each having a different degree of chromatic aberration.
In an appearance inspection method for detecting.a fault of an inspection object by comparing an image acquired by imaging a pattern arranged in a predetermined direction on the inspection object with an image arranged at a corresponding position of another inspection object, the second embodiment of the present invention provides an appearance inspection method, and an apparatus therefor, that comprise the steps of radiating rays of light by illumination means capable of radiating rays of light having at least two waveband lengths; causing the rays of light from the illumination means to pass through a first pin-hole array; changing the direction of the ray of light passing through the first pin-hole array by a beam splitter; causing the ray of light, the direction of which is so changed, to focus on the surface of the inspection object through an optical system having such chromatic aberration as to place the focus on a position corresponding to each wavelength band; separating the ray of light reflected from the inspection object into two directions by a beam splitter; causing one of the rays of light so separated to travel towards a first pin-hole array and the other of the rays of light to travel towards a second pin-hole array constituting a confocal microscope together with the first pin-hole array; receiving the ray of light passing through the second pin-hole array by imaging means to image an image of a pattern region; and detecting a fault of the inspection object on the basis of the comparison result obtained by comparing the image with an image of a pattern arranged at a corresponding position of other inspection object.
In this embodiment, too, imaging is conducted using the TDI camera.
The optical system having chromatic aberration comprises a plurality of objective lenses each having a different degree of chromatic aberration when images are acquired inside the device having the altitude difference such as between the cell portion and the peripheral circuit portion during a scanning operation of optical recognition means, the present invention allows each ray of light having a different wavelength band to always begin just-in-focus on each region. Since the image so acquired is a high quality image having a precise focus in each region, a high-precision fault detection capability can eventually be provided.